pollard



Jan. 24. 1956 c. E. POLLARD, JR

MEANS FOR PREVENTING CONTACT STICKING IN MERCURY CONTACT SWITCHES Filed Sept. 21, 1953 #vvavroa C. E. POLLARD, JR.

ATTORNEY United States Patent MEANS FOR PREVENTING CONTACT STICKING 1N MERCURY CONTACT SWITCHES Charles E. Pollard, Jr., Hohokus, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application September 21, 1953, Serial No. 381,449

6 Claims. (Cl. 200-112) This invention relates to electric switches and particularly to switches in which metal, in fluid form, is used as a medium through which circuit continuity is effected.

Switches of the general character described above may be of the solenoid type in which a circuit-controlling plunger moves under control of a magnetic field or by gravity or by both, into and out of engagement with a mercury mass; or of the mercury contact type in which the switch contacting surfaces are wetted, or continuously renewed by mercury, through capillary action, from a mercury pool or reservoir.

It has been discovered that switches in which mercury is employed as a contact medium have a tendency to stick. This tendency appears to increase with the increase in force between mating contacts or with increase in temperature. It has also been determined that relatively high temperatures coupled with long periods of contact closure tend to increase the sticking phenomenon to an objectionable degree.

it is the object of this invention to eliminate sticking of the contacts of mercury contact switches and to otherwise improve mercury type switches in general.

This object is attained in accordance with a feature of the invention by utilizing a solution of mercury and-copper as the fluid contact medium in a mercury type switch.

In accordance with another feature, a solution of mercury and copper is used in which the percentage of copper to mercury is such that, after wetting, substantially no copper-mercury amalgam is visible on the metal parts of the switch. More particularly, copper is added to the mercury in an amount slightly in excess of that which mercury can hold in solution, thus limiting the amount of free amalgam formed.

These and other features of the invention will be readily understood from the following detailed description when read with reference to the accompanying drawings in which:

Fig. l is a perspective drawing of a glass-sealed mercury type switch involving the features of the invention. In this view the glass envelope is broken away to show the interior construction and arrangement of the contact device;

Fig. 2 is a cross-essential view of a typical relay of which the switch unit of Fig. l is an essential component;

Fig. 3 is a sectional view taken along the line 3-3 of Fig. 2 looking in the direction of the small arrows;

Fig. 4 is an enlarged perspective view of the armature contact piece and the back contacts of the switch unit;

Fig. 5 is a conventional diagram indicating the connection of the winding of the relay shown in Fig. 2, and the armature and various contacts thereof to the pins of a conventional octal radio tube base;

Fig. 6 is a side view of a portion of the interior metal parts of the switch and illustrates one method of introducing the copper; and

Fig. 7 is a plan view of the switch parts shown in Fig. 6.

The switching device employed to disclose the invention consists of a glass envelope 1 into which a metal tube 2 2,732,459 Patented Jan. 24, 1956 is sealed at the bottom. This metal tube is for the usual purpose of exhausting the air from the envelope, introducing the mercury and later pumping the tube full of gas at high pressure.

While various methods may be resorted to in effecting the copper-mercury solution one may involve dropping a quantity of copper in the form of small pieces of copper wire into the tube 2 which may thereafter be crushed at its lower end and welded together. The short piece of wire 3 is conducting material which is used in the welding operation in order to produce the proper concentration of welding current and which itself becomes welded to the tube 2.

After the introduction of the copper, the tube is activated and the copper dissolves completely in the mercury.

As an alternate method of introducing the copper a small piece of copper wire 50 may be welded to the back of the metal piece 13 of the switch as shown in Figs. 6 and 7. This copper wire, during activation of the tube, is completely dissolved in the mercury.

Into the upper end of the glass tube 1 there are sealed four wires of magnetic material which form the exterior terminals of the device and to which contacts are secured within the envelope. The wires 4 and 5 constitute back contacts of the device and as shown in Fig. 4 these have pieces of contact metal 6 and 7, respectively, attached thereto. Similar wires 8 and 9 are used for the front contacts and are somewhat longer so as to provide a low reluctance circuit contiguous to the armature piece 10. The armature piece 10 is attached through the use of a light spring 11 to the metal tube 2. Other pieces of mercury-Wettable material 12 and 13 are also attache-d to the tube 2 so that the lower end of the armature 10 will move in a mass of the copper-mercury solution in such a way as to constitute a hydraulic brake in the manner disclosed in my United States Patent 2,406,036, granted August 20, 1946. The copper-mercury solution from the pool or reservoir 14 rises through capillary action through the space provided between the spring 11 and the plates 12 and 13 and further in the wick 41 made of two parallel wires having their upper ends bent into a T-shaped piece to form a contact cooperating with the two back contacts 6 and 7 and with the two front contacts similarly attached to the wires 8 and 9.

After the tube has been evacuated and the mercury and copper have been introduced, hydrogen is pumped into it to a pressure of 250 pounds to the square inch. It has been found that hydrogen at such high pressure raises the boiling point of mercury by a substantial amount so that heavier currents may be carried by this device than by a device having a pressure within a conventional degree.

In the relay used for disclosure purposes, a small piece of permanent magnetic material 15 is attached, as by soldering, to one of the front contact wires 9 and is in sulated from the other wire 8 by a small piece of insulation 16.

The switch as described is then inserted within a solenoid 17 and this assembly is then placed within a steel cover 18 of magnetic material so as to provide magnetic shielding for the relay. This assembly is then inverted and filled with a liquid plastic 25 such as hot wax which upon cooling will hold the parts of the relay rigid and inflexible. The tube is then attached to a conventional base 19 provided with the pins 20 and 21 and wired up according to the diagram of Fig. 5.

The relay selected for disclosure purposes in this application is further described and disclosed in United States Patent 2,577,602, issued December 4, 1951 to E. T. Burton.

While the ratio of copper to mercury is not a critical one and may vary within certain upper and lower limits, it

has been determined that contact sticking has been completely eliminated with substantially no amalgam show ing on the metal parts of the switch when an amount of copper is added which just saturates the mercury in the switch at 100 C.

The ratio of copper to'mercury has been investigated. In typical instances 0.43 to 0.56 milligram of copper have been added to.0.2 cubic centimeters of mercury. The lower figures, i. e., 0.43 milligram, is just about the amount which will saturate the mercury at 100 C., i. e., 0.014 per cent by weight. The lower limit of copper content useful for the purpose is not clearly defined and perhaps may be best stated as copper in an appreciable amount; 0.007 per cent of copper by weight appears to be adequate; the upper limit is determined by the point at which a loose or fiocculent amalgam not well incorporated in the main body of mercury-copper alloy begins to appear or tends to become deposited on the metal parts within the vessel, or both. This point is at 0.048 per cent copper by weight or in round numbers 0.05 per cent copper by weight. This may, therefore, be taken as the acceptable upper limit.

What is claimed is:

1. In an electric switch of the type employing engageable wetted contacts, an agent for Wetting the contacts comprising copper-mercury solution.

2. In an electric switch having engageable contacts housed in a sealed vitreous envelope, mercury, and copper dissolved in said mercury.

3. In an electric switch having movable circuit-controtling elements housed in a sealed vitreous envelope, and a pool of copper-mercury solution in said envelope.

4. An electric switch in which circuit continuities are made by way of contacts wetted by fluid contact material comprising copper to an amount up to 0.05 per cent by weight dissolved in mercury.

5. In an electric switch of the type employing disengageable circuit-controlling contacts of the mercurywetted type, means for eliminating the tendency said contacts have to adhere comprising copper to an amount up to 0.05 per cent by weight dissolved in the wetting mercury.

6. In an electric switch having metal components including engageable wetted contact-bearing members, an agent for wetting the contacts comprising a solution of copper and mercury in which the percentage of copper is such that, after wetting, the metal components are substantially free of copper-mercury amalgam.

References Cited in the file of this patent UNITED STATES PATENTS 

1. IN AN ELECTRIC SWITCH OF THE TYPE EMPLOYING ENGAGEABLE WETTED CONTACTS, AN AGENT FOR WETTING THE CONTACTS COMPRISING COPPER-MERCURY SOLUTION. 